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Abstract

For Fourier domain optical coherence tomography any sample movement during camera integration causes blurring of interference fringes and as such reduction of sensitivity for flow detection. The proposed method overcomes this problem by phase-matching a reference signal to the sample motion. The interference fringes corresponding to flow signal will appear frozen across the detector whereas those of static sample structures will be blurred resulting in enhanced contrast for blood vessels. An electro-optic phase modulator in the reference arm, driven with specific phase cycles locked to the detection frequency, allows not only for qualitative but also for quantitative flow detection already from the relative signal intensities. First applications to extract in-vivo retinal flow and to visualize 3D vascularization, i.e. optical vivisection, are presented.

Fig. 2. Normalized signal attenuation A as a function of Δz, the displacement of a sample surface during the integration time, and z, the axial coordinate in dB. The three plots correspond to a ΔkFWHM of 0.2/μm, 1/μm and 2/μm. For larger spectral bandwidths, the signal decays faster with Δz and the splitting occurs earlier.

Fig. 3. (a). Maximum value in z-direction of the normalized signal attenuation A as a function of displacement Δz for the same parameters as in Fig. 2. (b) A as a function of sample velocity for the static case (solid line) and with a reference velocity offset VR (dashed line). As indicated, the reference velocity offset reduces static structure intensity (green arrow) and enhances moving structure intensity (red arrow).

Fig. 4. (a). Signal attenuation A for the static (solid line) and shifted case (dashed line). The red shaded region indicates the intersection of the main lobes. The green dash-dotted line indicates the MPA for a typical local signal in the tomogram. (b) Velocity as a function of the quotient ΔA of the shifted and the static case within the red shaded area (red solid line). The curve allows associating to a given ΔA value a unique sample velocity VS. The velocity error δV [Eq. (14)] can be read from the dashed line. Within the green dash-dotted lines it is possible attributing a unique velocity value (see text for details).

Fig. 9. Velocity determination by unambiguous differential velocity mapping (see §2.3). For each differential image (lhs) a corresponding theoretical ΔA curve (rhs) is shown. The blue and red solid lines correspond to the selected vessels on the lhs. The horizontal lines and circles on the rhs indicate possible velocity values according to the intensity level in the corresponding tomogram. The ambiguity is removed by looking for the common velocity in all three differential maps as indicated by the respective vertical lines on the rhs. The green dashed lines indicate the MPA limit of -18.4dB. Tomogram size: 3mm x 1.65mm (lateral x depth) with 8.5μm axial resolution (in air).